1. Field of the Invention
The present invention relates to a computed tomography (“CT”) apparatus, and more particularly to the image reconstruction method of cone-beam helical CT wherein an X-ray source has a helical trajectory.
2. Description of the Related Art
In recent years, in the field of X-ray CT, basic image reconstruction algorithms have been continuously developed and variously proposed for the practicability of three-dimensional (3D) image display. In, for example, so-called “cone-beam helical CT” wherein an X-ray source has a helical trajectory, a large number of approximate reconstruction techniques have been proposed. The techniques include what is called “TCOT (True COne beam Tomography reconstruction algorithm) method”, a helical oblique section reconstruction method (also called “ASSR method”), etc. These reconstruction techniques obtain approximate solutions to the last, but the existence of an exact solution has recently been demonstrated in the cone-beam helical CT.
The researches and developments, feasibilities, etc. of the image reconstruction algorithms of the CT will be outlined in the order of (1) helical CT, (2) multi-slice helical CT based on fan-beam geometry, (3) a scheme for obtaining an approximate solution in 3D helical CT based on cone-beam geometry, and (4) a scheme for obtaining an exact solution in the 3D helical CT based on the cone-beam geometry.
(1) Helical CT
In helical CT, a diagnostic table is moved in synchronism with the rotation of an X-ray source (source) as well as a detector, whereby the X-ray source is caused to depict a helical motion relatively to a subject or patient, virtual projection data corresponding to any slice positions designated between adjacent helices are created in succession usually by linear interpolations, and the image of the subject is reconstructed on the basis of the virtual projection data. In effect, however, only one slice is substantially obtained per revolution. By way of example, if 2 mm-slice data are to be created for a region which has a thickness corresponding to 100 mm, radiographing operations of 50 revolutions are required.
(2) Multi-Slice Helical CT Based on Fan-Beam Geometry
As the expansion of the helical CT, there has been known multi-slice helical CT based on a scheme wherein a detector is constructed in the form of 2–4 channels in a slicing direction. According to the multi-slice helical CT, a data acquisition rate becomes 2–4 times as high as in the ordinary helical CT.
In practical use, with the detector having 4 channels or so in the slicing direction, it has been verified that, even when individual projection data obtained by the different channels are regarded as parallel beams which are parallel to the slicing direction, namely, multilayer two-dimensional fan beams which are based on fan-beam geometry, they are little problematic in the reconstruction thereof. Such a scheme has already been put into practical use.
(3) Scheme for Obtaining Approximate Solution in 3D Helical CT Based on Cone-Beam Geometry
When the number of channels of the detector in the multi-slice helical CT is further increased from 2–4 to 8 or 16, individual projection data obtained by the different channels can no longer be regarded as the parallel beams based on the fan-beam geometry, and cone-beam geometry needs to be considered.
A first solution based on the cone-beam geometry in this case is a helical oblique section reconstruction method proposed in JP-A-8-187240 by the inventor. Also, a technique called “ASSR method” (Kachelriess: Med. Phys., 27, 754–772) is substantially equivalent to the helical oblique section reconstruction method.
Besides, what is called “TCOT method” has been proposed (refer to U.S. Pat. No. 5,825,842) as a method wherein a Feldkamp method which is originally a reconstruction technique in the case where a source has a circular trajectory is applied to helical scan.
Any of these techniques is an approximation technique, and artifacts become conspicuous especially in a case where the number of channels in the slicing direction further increases from 8–16 to 32, 64, . . . . It has therefore been required to attain a still higher precision.
(4) Scheme for Obtaining Exact Solution in 3D Helical CT Based on Cone-beam Geometry
On the other hand, it has been demonstrated in recent years that, in a case where a source has a trajectory such as helical trajectory, an exact solution is theoretically existent even for a long and large object and even in a smooth functional system. Regarding the existence of the exact solution, some demonstrations have been given as stated in, for example, Schaller et al.: “Exact Radon rebinning algorithm for long object problem in helicalcone-beamCT”, IEEE Trans. Med. Imag. 19361–75 (2000).
Approaches to the developments of image reconstruction algorithms having heretofore been made, however, have had the problem that, although the exact solution is concerned, the quantity of computations is large, so degradation ascribable to interpolation processing is liable to occur in case of employing actual discrete data. There has also been the problem that, since the range of data necessary for obtaining a certain slice image is wide, the data are easily influenced by the peripheral tissues of a subject and the temporal fluctuation thereof, so they are not satisfactory for use in a medical image diagnostic equipment.
Therefore, further improvement is desired. In particular, in three-dimensional CT whose practical use has been started at present, especially in 3D helical CT based on cone-beam geometry, it is desired to develop a new practicable technique which employs only data having a still higher precision and being necessary and sufficient to the utmost. Nevertheless, it is an actual situation that the precisions of approximations by techniques hitherto proposed cannot be said satisfactory, and that artifacts remain yet.